1. Field of the Invention
The present invention relates to highly stable frequency sources and more particularly, to an improved voltage controlled crystal oscillator particularly adaptable for being stabilized by an atomic frequency standard.
2. Prior Art
Although the present invention may find practical application as a stand-alone oscillator or as an atomic frequency standard stabilized oscillator in any one of numerous atomic stabilized frequency sources, it is particularly adaptable for operation in a rubidium vapor cell frequency standard. Rubidium vapor cell frequency standards as well as other types of atomic stabilized frequency sources are described extensively in the literature. For example, reference may be had to the texts respectively entitled "Frequency and Time" by P. Kartaschoff, Academic Press, 1978; and "Frequency Synthesizers Theory and Design", Second Edition, by Vadim Manassewitsch, John Wiley and Sons, 1980. Such frequency sources are stabilized by quantum mechanical atomic state transition resonances such as the hyperfine atomic resonance frequency related to a change in the internal energy of the atom. A rubidium frequency standard operates as a discriminator based upon the energy absorption characteristic of rubidium-87. In practice, a rubidium lamp passes a light beam into a rubidium absorption cell. The rubidium cell absorbs some of the light energy because of the energy level transitions in the rubidium-87 gas. When an electromagnetic field of frequency equal to the resonance frequency of the rubidium vapor is applied to the vapor cell, the number of energy level transitions in the rubidium-87 gas is increased and more of the light emitted by the rubidium lamp is absorbed by the rubidium vapor cell. Typically, a photodiode is used to detect the occurrence of the maximum absorption of light from the rubidium lamp which occurs when the frequency of the excitation electromagnetic field exactly matches the rubidium resonance frequency. Typically, a frequency synthesizer is used to generate the appropriate electromagnetic field frequency of approximately 6,834.685 MHz. This field is modulated at a relatively slow rate (i.e., 154 Hz.) so that the photodiode provides a demodulated signal which may be applied to a phase detector or comparator which also receives the reference modulation signal. The output of the phase comparator is a DC error voltage which is used to control a voltage controlled crystal oscillator at a selected frequency, typically of 5 or 10 MHz. In this manner, the frequency of the crystal oscillator is stabilized to approximately one part in 10.sup.11 or better over long periods of time to provide a highly stable and accurate frequency source.
The elaborate design and complexity and commensurate costs for an atomic frequency standard for stabilizing a crystal oscillator would be for nought unless the oscillator itself were fairly stable to begin with. The oscillator should be relatively insensitive to temperature and mechanical effects and to variations in the load to which the crystal output is connected. In order to achieve this commensurate crystal oscillator stability and insensitivity in the prior art, it has been necessary to resort to elaborate electrical and mechanical means for isolating the crystal oscillator circuit to render it insensitive to physical and thermal shock. In addition, it has been necessary to use relatively expensive crystals which have a low effective series resistance and good aging characteristics. Unfortunately, such elaborate measures and expensive crystals substantially increase the design and production costs of the crystal oscillator and of the overall frequency standard system, and thus contribute significantly to the prohibitive costs typical for prior art atomic clock stabilized oscillators.